# Tuple

In mathematics, a **tuple** is a finite ordered list (sequence) of elements. An **n-tuple** is a sequence (or ordered list) of n elements, where n is a non-negative integer. There is only one 0-tuple, referred to as *the empty tuple*. An n-tuple is defined inductively using the construction of an ordered pair.

In computer science, tuples come in many forms. Most typed functional programming languages implement tuples directly as product types,^{[1]} tightly associated with algebraic data types, pattern matching, and destructuring assignment.^{[2]} Many programming languages offer an alternative to tuples, known as record types, featuring unordered elements accessed by label.^{[3]} A few programming languages combine ordered tuple product types and unordered record types into a single construct, as in C structs and Haskell records. Relational databases may formally identify their rows (records) as *tuples*.

Tuples also occur in relational algebra; when programming the semantic web with the Resource Description Framework (RDF); in linguistics;^{[4]} and in philosophy.^{[5]}

The term originated as an abstraction of the sequence: single, couple/double, triple, quadruple, quintuple, sextuple, septuple, octuple, ..., *n*‑tuple, ..., where the prefixes are taken from the Latin names of the numerals. The unique 0-tuple is called the null tuple or empty tuple. A 1‑tuple is called a single (or singleton), a 2‑tuple is called an ordered pair or couple, and a 3‑tuple is called a triple (or triplet). The number *n* can be any nonnegative integer. For example, a complex number can be represented as a 2‑tuple of reals, a quaternion can be represented as a 4‑tuple, an octonion can be represented as an 8‑tuple, and a sedenion can be represented as a 16‑tuple.

Although these uses treat *‑uple* as the suffix, the original suffix was *‑ple* as in "triple" (three-fold) or "decuple" (ten‑fold). This originates from medieval Latin *plus* (meaning "more") related to Greek ‑πλοῦς, which replaced the classical and late antique *‑plex* (meaning "folded"), as in "duplex".^{[6]}^{[a]}

There are several definitions of tuples that give them the properties described in the previous section.

Another way of modeling tuples in Set Theory is as nested ordered pairs. This approach assumes that the notion of ordered pair has already been defined.

A variant of this definition starts "peeling off" elements from the other end:

Using Kuratowski's representation for an ordered pair, the second definition above can be reformulated in terms of pure set theory:

In discrete mathematics, especially combinatorics and finite probability theory, *n*-tuples arise in the context of various counting problems and are treated more informally as ordered lists of length *n*.^{[7]} *n*-tuples whose entries come from a set of *m* elements are also called *arrangements with repetition*, *permutations of a multiset* and, in some non-English literature, *variations with repetition*. The number of *n*-tuples of an *m*-set is *m*^{n}. This follows from the combinatorial rule of product.^{[8]} If *S* is a finite set of cardinality *m*, this number is the cardinality of the *n*-fold Cartesian power *S* × *S* × ⋯ × *S*. Tuples are elements of this product set.

In type theory, commonly used in programming languages, a tuple has a product type; this fixes not only the length, but also the underlying types of each component. Formally:

The tuple with labeled elements used in the relational model has a record type. Both of these types can be defined as simple extensions of the simply typed lambda calculus.^{[9]}

The *n*-tuple of type theory has the natural interpretation as an *n*-tuple of set theory:^{[10]}